Using eddy current test systems for non-destructive testing of parts formed of conductive materials is common in a variety of environments. The testing applications include but are not limited to alloy sorting, detecting cracks in air frames and engines, and remotely testing structures in hazardous environments, such as tubing in nuclear power plant heat exchanger units. Such testing applications are frequently essential for controlling the quality of the manufactured part and preventing potentially catastrophic failures of structural members, weldments, and other joints.
Typically, an eddy current test system consists of an eddy current probe coupled to a display. The display provides a visual read out of the eddy current signal strength generated in the conductive part. The eddy current signal strength varies depending on near surface defects or flaws detected in the material of the conductive part.
The eddy current test system provides an alternating voltage to a drive coil which is part of the eddy current probe. The drive coil is brought into contact with the Part Under Test ("PUT") made of a conductive material. The alternating voltage present in the drive coil induces an electromagnetic field in the PUT. The electromagnetic field, in turn, generates eddy currents in the conductive part. Thus, eddy currents exist as a result of voltages induced in the body of the conductive PUT by variations of the magnetic flux. The variation of the magnetic flux is the result of a varying magnetic field or of the relative motion of the conductive part with respect to the magnetic field.
A receive coil is typically positioned in close proximity to the drive coil. The receive coil receives the eddy currents and sends the signals to the test system display for displaying the eddy current signal strength in suitable units. A near surface flaw or anomaly in the conductive part changes the electromagnetic field and consequently, the eddy current signal strength, received by the receive coil. Although the change in the electromagnetic field in the conductive part is commonly weak and difficult to repeatably measure, the change in the eddy current signal strength is much more dramatic and easily and repeatably measurable.
There are several problems associated with known eddy current test systems. An important requirement in eddy current testing is that the drive and/or receive coil remain in close contact with the PUT to maximize the electromagnetic field induced in the PUT. Often, the drive coil and/or the receive coil lifts off the PUT as the eddy current test probe traverses the PUT. If either coil lifts off the PUT, the electromagnetic field injected into the part is dramatically weakened, reducing the eddy current signal strength received by the receive coil. This reduction in the received eddy current signal strength can falsely signal a defect or anomaly in the PUT.
Moreover, traditional eddy current test systems include sensor assemblies having fixed-shaped drive and receive coils. Fixed-shaped coils do not maximize surface contact with the PUT because they do not flex to accommodate differently shaped PUTs. Attempts to maximize surface contact with the PUT have produced multiple coil arrays. U.S. Pat. No. 5,389,876, to Hedengren et al., issued Feb. 14, 1995, describes a multiple coil array eddy current test system. The multiple coil array described in Hedengren provides a degree of coil flexibility not found in fixed-coil assemblies. However, the Hedengren test system requires a spatially correlated coil array of substantially identical probe elements which, in turn, necessitates tightly controlled and complex manufacturing processes. Moreover, multiple coil arrays such as the coil array described in Hedengren require stringent and involved electrical connections.
Accordingly, a need remains for an eddy current test probe having a sensor assembly which is flexible, maximizes surface contact with the PUT, prevents coil lift off, is easy to manufacture, and simplifies the required electrical connections. A need also remains for an eddy current test probe that is repeatable, accurate, reliable, and easy to use.